DACTHAL is a trademark of the Fermenta Plant Protection Company, Mentor, Ohio, for a composition of dimethyl ester 2,3,5,6-tetrachloroterephthalic acid, commonly known as DCPA, used in pre-emergence weed control. As a result of its favorable crop tolerance properties, DACTHAL's major areas of use are in vegetables and turf. DACTHAL is applied to the soil prior to the emergence of the crops (and/or weeds) above the soil line. The DACTHAL then kills or interferes with the life cycle of the weeds, to increase the yield and/or quality of the crops.
DACTHAL is produced by Fermenta, and others, through a series of photochlorination, fusion, thermal chlorination, esterification and distillation reactions. In the process for producing DACTHAL, sodium chloride, and organic salts, such as sodium salts of tetrachloroterephthalic acid, are produced as by-products. However, since the sodium chloride and tetrachloroterephthalic acid salt by-products generate many harmful effects on crops and foliage, it is desirous to remove the salt by-products from the DACTHAL solution.
The conventional method of desalinating the DACTHAL solutions concerns the use of a standard basket centrifuge to separate the waste salt solids from a solution of DACTHAL (product) and xylene (solvent). Centrifugal separation is a mechanical means of separating the components of a mixture by accelerating the material in a centrifugal field. Centrifugal separation of a mixture of immiscible components makes use of either density differences between the components or drainage of a liquid phase through a packed bed or cake of solid particles.
In the preferred conventional method for separating the waste salt solids from the DACTHAL solution, the salt solids are removed by centrifugation in a steam jacketed "Sharples" (The Sharples Corp.) centrifuge with a 48" diameter .times. 30" MONEL (trademark of Huntington Alloys, Inc., Alloys International, Inc. for a large group of corrosion-resistant alloys of predominantly nickel and copper) basket and a 40 HP, 1750 RPM Oilgear hydraulic drive unit. A plow and a plow stop mechanism for precoat usage is provided in the centrifuge. A precoat or filter aid media such as diatomaceous earth (J.M. Celite AFII Precoat and/or FilterMedia Precoat) is deposited approximately 2.5 to 3.0 inches in depth, on a fine mesh screen lining the centrifuge basket by standard procedure utilizing the dual inflow feed pipe of the Sharples centrifuge. Precoating improves the performance and efficiency of the centrifuge substantially and the quality of the precoat layer is an important variable in determining centrifuge performance.
A slurry consisting of the DACTHAL product, the xylene solvent and the waste solids (including the harmful salt by-products generated in the DACTHAL production process) is then fed (sprayed tangentially) through the dual inflow feed pipe into the spinning centrifuge basket (500-700 rpm) containing the precoat, where the solids are deposited on the surface of the filter media, while the liquid DACTHAL-xylene solution penetrates the media and exits via perforations in the basket and is collected in the outside chamber of the centrifuge. The feed rate and feed cycle times of the slurry vary depending upon the nature of the precoat which deteriorates with successive feed cycles, and the quality of the slurry.
The liquid is then drained from the centrifuge chamber and is routed to the centrifuge product tank. From the centrifuge product tank, the DACTHAL-xylene solution is pumped through a pressure leaf filter and a polishing filter to remove trace amounts of salt before going into a stripper feed tank. The solution is then pumped from the stripper feed tank to a xylene stripper to separate the xylene from the DACTHAL, resulting in a desalinated and purified end-product.
Following the feed cycle, the speed of the centrifuge basket is increased (650-1000 rpm) to remove as much of the residual xylene-DACTHAL solution from the wet cake as possible, before the solvent wash cycle. The length of this spin period varies depending upon the amount of time required to obtain a cake surface free of mother liquor from the slurry solution.
Upon completion of the spin cycle, hot xylene (at about 120.degree. C.) is sprayed on to the cake while the basket is still spinning (650-1000 rpm) to displace and dissolve (thereby recovering) most of the DACTHAL in the salt cake. A second spin cycle (650-1000 rpm) then follows to remove any residual xylene from the previous two cycles. The second spin cycle continues until there is no free liquor in the bottom of the basket and the cake appears to be smooth and dry.
After the second spin cycle, the spin of the centrifuge basket is decreased from about 650-1000 rpm to about 40-60 rpm for the unload cycle. During this cycle the unloader (plow) slowly moves into the salt cake, removing the cake and a small portion of the precoat until the plow stop is reached. The cake and precoat removed by the plow fall from the centrifuge basket through a salt chute into a hot oil jacketed, double screw salt cake conveyor/vaporizer. The screw is heated to vaporize and remove xylene from the salt and precoat before the salt and precoat are dumped and disposed of. The xylene solvent is then recycled for use in the washing cycle.
Upon completion of the unload cycle, the machine automatically returns to the acceleration cycle and the feed cycle begins anew to produce an additional batch of desalinated DACTHAL.
The above cycles are repeated and new batches of desalinated DACTHAL are produced until new precoat needs to be added or replaced. The position of the plow stop is altered between every other batch to allow successively deeper penetration of the plow blade into the precoat. While plowing renews the surface of the filter media, it also eventually depletes the supply of media, necessitating fresh precoat application. New precoat is applied after approximately twelve hours of continuous operations, and once every 24 hours the entire precoat layer is replaced by standard methods.
Although the above conventional method of removing the harmful salt by-products of a DACTHAL slurry has been effective in producing desalinated DACTHAL solutions, the following limitations have been noted:
A. Throughput
1. Centrifuge feed capacity is limited to the drain rate of solution through the filter media. This drain rate is determined by media condition, which deteriorates with successive feed cycles, and slurry quality variability.
2. Limited feed rates prolong the required duration of the feed portion of a centrifuge cycle. Normally transport rates through the filter cake decrease, with increased cake depth. Some efficiency loss therefore occurs as prolonged feed periods incur diminishing solution throughput. The diminishing return of extended feed periods thus limits total capacity.
3. Erratic feed capacities force limitation of feed rates to non-optimal performance according to "average" operating conditions or "worst case" operating conditions.
4. The conventional method dictates plow-out of the centrifuge before a basket is full. This reduces overall throughput.
B. Waste Disposal
1. The conventional method dictates a reduced slurry feed rate. The reduced velocity of liquid flow across the face of the centrifuge basket allows segmentation of solids according to mass and particle size. Coarse particles settle first, close to the feed nozzle. Finer particles flow upward and form a film that decreases in porosity with distance from the feed point. This non-porous band becomes impenetratable to rinse solvent and actually seals the surface of the filter media, disallowing proper draining of the filter media. Solids dropped from the basket are therefore wet with solvent and contain high concentrations of product. Significant costs are incurred to enable disposal of solids laden with particular solvents and product loss is an additional cost factor.
C. Product Quality
1. The conventional method exposed the operation to periodic overflow of the centrifuge basket. During the normal course of operation of the conventional method, salt gradually permeates the filter media, partially plugging it off and reducing the drain rate of the slurry, (see above). This, in turn, increases the tendency of the basket to overflow during the feed cycle, thereby increasing the amount of insoluble solids in the filtrate. Solids passing over the top of the basket required removal in subsequent filtration systems. This decreases efficiency, adds equipment and personnel costs and increases personnel chemical exposure. (This is due to operation of subsequent filtration systems).
It has now been found that the limitations of the above described conventional method and apparatus for desalinating DACTHAL can be overcome by the present invention.